Problem: A jar contains $3$ red marbles, $5$ green marbles, and $10$ blue marbles. If a marble is randomly chosen, what is the probability that it is not green?
Solution: There are $3 + 5 + 10 = 18$ marbles in the jar. There are $5$ green marbles. That means $18 - 5 = 13$ are not green. The probability is $ \frac{13}{18}$.